[PATCH] srcu-3: RCU variant permitting read-side blocking
Updated patch adding a variant of RCU that permits sleeping in read-side critical sections. SRCU is as follows: o Each use of SRCU creates its own srcu_struct, and each srcu_struct has its own set of grace periods. This is critical, as it prevents one subsystem with a blocking reader from holding up SRCU grace periods for other subsystems. o The SRCU primitives (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu()) all take a pointer to a srcu_struct. o The SRCU primitives must be called from process context. o srcu_read_lock() returns an int that must be passed to the matching srcu_read_unlock(). Realtime RCU avoids the need for this by storing the state in the task struct, but SRCU needs to allow a given code path to pass through multiple SRCU domains -- storing state in the task struct would therefore require either arbitrary space in the task struct or arbitrary limits on SRCU nesting. So I kicked the state-storage problem up to the caller. Of course, it is not permitted to call synchronize_srcu() while in an SRCU read-side critical section. o There is no call_srcu(). It would not be hard to implement one, but it seems like too easy a way to OOM the system. (Hey, we have enough trouble with call_rcu(), which does -not- permit readers to sleep!!!) So, if you want it, please tell me why... [josht@us.ibm.com: sparse notation] Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com> Signed-off-by: Josh Triplett <josh@freedesktop.org> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
This commit is contained in:
Paul E. McKenney
Linus Torvalds
parent
95d77884c7
commit
621934ee7e
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@ -221,3 +221,41 @@ over a rather long period of time, but improvements are always welcome!
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disable irq on a given acquisition of that lock will result in
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deadlock as soon as the RCU callback happens to interrupt that
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acquisition's critical section.
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13. SRCU (srcu_read_lock(), srcu_read_unlock(), and synchronize_srcu())
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may only be invoked from process context. Unlike other forms of
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RCU, it -is- permissible to block in an SRCU read-side critical
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section (demarked by srcu_read_lock() and srcu_read_unlock()),
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hence the "SRCU": "sleepable RCU". Please note that if you
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don't need to sleep in read-side critical sections, you should
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be using RCU rather than SRCU, because RCU is almost always
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faster and easier to use than is SRCU.
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Also unlike other forms of RCU, explicit initialization
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and cleanup is required via init_srcu_struct() and
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cleanup_srcu_struct(). These are passed a "struct srcu_struct"
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that defines the scope of a given SRCU domain. Once initialized,
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the srcu_struct is passed to srcu_read_lock(), srcu_read_unlock()
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and synchronize_srcu(). A given synchronize_srcu() waits only
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for SRCU read-side critical sections governed by srcu_read_lock()
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and srcu_read_unlock() calls that have been passd the same
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srcu_struct. This property is what makes sleeping read-side
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critical sections tolerable -- a given subsystem delays only
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its own updates, not those of other subsystems using SRCU.
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Therefore, SRCU is less prone to OOM the system than RCU would
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be if RCU's read-side critical sections were permitted to
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sleep.
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The ability to sleep in read-side critical sections does not
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come for free. First, corresponding srcu_read_lock() and
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srcu_read_unlock() calls must be passed the same srcu_struct.
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Second, grace-period-detection overhead is amortized only
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over those updates sharing a given srcu_struct, rather than
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being globally amortized as they are for other forms of RCU.
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Therefore, SRCU should be used in preference to rw_semaphore
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only in extremely read-intensive situations, or in situations
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requiring SRCU's read-side deadlock immunity or low read-side
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realtime latency.
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Note that, rcu_assign_pointer() and rcu_dereference() relate to
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SRCU just as they do to other forms of RCU.
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@ -45,7 +45,8 @@ o How can I see where RCU is currently used in the Linux kernel?
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Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
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"rcu_read_lock_bh", "rcu_read_unlock_bh", "call_rcu_bh",
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"synchronize_rcu", and "synchronize_net".
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"srcu_read_lock", "srcu_read_unlock", "synchronize_rcu",
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"synchronize_net", and "synchronize_srcu".
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o What guidelines should I follow when writing code that uses RCU?
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@ -778,6 +778,8 @@ Markers for RCU read-side critical sections:
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rcu_read_unlock
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rcu_read_lock_bh
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rcu_read_unlock_bh
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srcu_read_lock
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srcu_read_unlock
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RCU pointer/list traversal:
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@ -804,6 +806,7 @@ RCU grace period:
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synchronize_net
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synchronize_sched
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synchronize_rcu
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synchronize_srcu
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call_rcu
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call_rcu_bh
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@ -0,0 +1,49 @@
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/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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struct srcu_struct_array {
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int c[2];
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};
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struct srcu_struct {
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int completed;
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struct srcu_struct_array *per_cpu_ref;
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struct mutex mutex;
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};
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#ifndef CONFIG_PREEMPT
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#define srcu_barrier() barrier()
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#else /* #ifndef CONFIG_PREEMPT */
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#define srcu_barrier()
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#endif /* #else #ifndef CONFIG_PREEMPT */
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void init_srcu_struct(struct srcu_struct *sp);
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void cleanup_srcu_struct(struct srcu_struct *sp);
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int srcu_read_lock(struct srcu_struct *sp) __acquires(sp);
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void srcu_read_unlock(struct srcu_struct *sp, int idx) __releases(sp);
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void synchronize_srcu(struct srcu_struct *sp);
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long srcu_batches_completed(struct srcu_struct *sp);
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void cleanup_srcu_struct(struct srcu_struct *sp);
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@ -8,7 +8,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o profile.o \
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signal.o sys.o kmod.o workqueue.o pid.o \
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rcupdate.o extable.o params.o posix-timers.o \
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kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
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hrtimer.o rwsem.o latency.o nsproxy.o
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hrtimer.o rwsem.o latency.o nsproxy.o srcu.o
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obj-$(CONFIG_STACKTRACE) += stacktrace.o
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obj-y += time/
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@ -0,0 +1,257 @@
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/*
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* Sleepable Read-Copy Update mechanism for mutual exclusion.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
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*
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* Copyright (C) IBM Corporation, 2006
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*
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* Author: Paul McKenney <paulmck@us.ibm.com>
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*
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* For detailed explanation of Read-Copy Update mechanism see -
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* Documentation/RCU/ *.txt
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*
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*/
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#include <linux/module.h>
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#include <linux/mutex.h>
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#include <linux/percpu.h>
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#include <linux/preempt.h>
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#include <linux/rcupdate.h>
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#include <linux/sched.h>
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#include <linux/slab.h>
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#include <linux/smp.h>
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#include <linux/srcu.h>
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/**
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* init_srcu_struct - initialize a sleep-RCU structure
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* @sp: structure to initialize.
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*
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* Must invoke this on a given srcu_struct before passing that srcu_struct
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* to any other function. Each srcu_struct represents a separate domain
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* of SRCU protection.
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*/
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void init_srcu_struct(struct srcu_struct *sp)
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{
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sp->completed = 0;
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sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
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mutex_init(&sp->mutex);
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}
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/*
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* srcu_readers_active_idx -- returns approximate number of readers
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* active on the specified rank of per-CPU counters.
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*/
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static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
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{
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int cpu;
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int sum;
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sum = 0;
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for_each_possible_cpu(cpu)
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sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
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return sum;
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}
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/**
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* srcu_readers_active - returns approximate number of readers.
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* @sp: which srcu_struct to count active readers (holding srcu_read_lock).
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*
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* Note that this is not an atomic primitive, and can therefore suffer
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* severe errors when invoked on an active srcu_struct. That said, it
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* can be useful as an error check at cleanup time.
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*/
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int srcu_readers_active(struct srcu_struct *sp)
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{
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return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
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}
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/**
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* cleanup_srcu_struct - deconstruct a sleep-RCU structure
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* @sp: structure to clean up.
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*
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* Must invoke this after you are finished using a given srcu_struct that
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* was initialized via init_srcu_struct(), else you leak memory.
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*/
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void cleanup_srcu_struct(struct srcu_struct *sp)
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{
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int sum;
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sum = srcu_readers_active(sp);
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WARN_ON(sum); /* Leakage unless caller handles error. */
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if (sum != 0)
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return;
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free_percpu(sp->per_cpu_ref);
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sp->per_cpu_ref = NULL;
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}
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/**
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* srcu_read_lock - register a new reader for an SRCU-protected structure.
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* @sp: srcu_struct in which to register the new reader.
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*
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* Counts the new reader in the appropriate per-CPU element of the
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* srcu_struct. Must be called from process context.
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* Returns an index that must be passed to the matching srcu_read_unlock().
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*/
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int srcu_read_lock(struct srcu_struct *sp)
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{
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int idx;
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preempt_disable();
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idx = sp->completed & 0x1;
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barrier(); /* ensure compiler looks -once- at sp->completed. */
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per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
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srcu_barrier(); /* ensure compiler won't misorder critical section. */
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preempt_enable();
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return idx;
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}
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/**
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* srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
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* @sp: srcu_struct in which to unregister the old reader.
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* @idx: return value from corresponding srcu_read_lock().
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*
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* Removes the count for the old reader from the appropriate per-CPU
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* element of the srcu_struct. Note that this may well be a different
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* CPU than that which was incremented by the corresponding srcu_read_lock().
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* Must be called from process context.
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*/
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void srcu_read_unlock(struct srcu_struct *sp, int idx)
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{
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preempt_disable();
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srcu_barrier(); /* ensure compiler won't misorder critical section. */
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per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
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preempt_enable();
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}
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/**
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* synchronize_srcu - wait for prior SRCU read-side critical-section completion
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* @sp: srcu_struct with which to synchronize.
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*
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* Flip the completed counter, and wait for the old count to drain to zero.
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* As with classic RCU, the updater must use some separate means of
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* synchronizing concurrent updates. Can block; must be called from
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* process context.
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*
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* Note that it is illegal to call synchornize_srcu() from the corresponding
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* SRCU read-side critical section; doing so will result in deadlock.
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* However, it is perfectly legal to call synchronize_srcu() on one
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* srcu_struct from some other srcu_struct's read-side critical section.
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*/
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void synchronize_srcu(struct srcu_struct *sp)
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{
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int idx;
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idx = sp->completed;
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mutex_lock(&sp->mutex);
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/*
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* Check to see if someone else did the work for us while we were
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* waiting to acquire the lock. We need -two- advances of
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* the counter, not just one. If there was but one, we might have
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* shown up -after- our helper's first synchronize_sched(), thus
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* having failed to prevent CPU-reordering races with concurrent
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* srcu_read_unlock()s on other CPUs (see comment below). So we
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* either (1) wait for two or (2) supply the second ourselves.
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*/
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if ((sp->completed - idx) >= 2) {
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mutex_unlock(&sp->mutex);
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return;
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}
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synchronize_sched(); /* Force memory barrier on all CPUs. */
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/*
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* The preceding synchronize_sched() ensures that any CPU that
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* sees the new value of sp->completed will also see any preceding
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* changes to data structures made by this CPU. This prevents
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* some other CPU from reordering the accesses in its SRCU
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* read-side critical section to precede the corresponding
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* srcu_read_lock() -- ensuring that such references will in
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* fact be protected.
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*
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* So it is now safe to do the flip.
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*/
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idx = sp->completed & 0x1;
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sp->completed++;
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synchronize_sched(); /* Force memory barrier on all CPUs. */
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/*
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* At this point, because of the preceding synchronize_sched(),
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* all srcu_read_lock() calls using the old counters have completed.
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* Their corresponding critical sections might well be still
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* executing, but the srcu_read_lock() primitives themselves
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* will have finished executing.
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*/
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while (srcu_readers_active_idx(sp, idx))
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schedule_timeout_interruptible(1);
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synchronize_sched(); /* Force memory barrier on all CPUs. */
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/*
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* The preceding synchronize_sched() forces all srcu_read_unlock()
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* primitives that were executing concurrently with the preceding
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* for_each_possible_cpu() loop to have completed by this point.
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* More importantly, it also forces the corresponding SRCU read-side
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* critical sections to have also completed, and the corresponding
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* references to SRCU-protected data items to be dropped.
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*
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* Note:
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*
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* Despite what you might think at first glance, the
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* preceding synchronize_sched() -must- be within the
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* critical section ended by the following mutex_unlock().
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* Otherwise, a task taking the early exit can race
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* with a srcu_read_unlock(), which might have executed
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* just before the preceding srcu_readers_active() check,
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* and whose CPU might have reordered the srcu_read_unlock()
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* with the preceding critical section. In this case, there
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* is nothing preventing the synchronize_sched() task that is
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* taking the early exit from freeing a data structure that
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* is still being referenced (out of order) by the task
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* doing the srcu_read_unlock().
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*
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* Alternatively, the comparison with "2" on the early exit
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* could be changed to "3", but this increases synchronize_srcu()
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* latency for bulk loads. So the current code is preferred.
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*/
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mutex_unlock(&sp->mutex);
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}
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/**
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* srcu_batches_completed - return batches completed.
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* @sp: srcu_struct on which to report batch completion.
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*
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* Report the number of batches, correlated with, but not necessarily
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* precisely the same as, the number of grace periods that have elapsed.
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*/
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long srcu_batches_completed(struct srcu_struct *sp)
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{
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return sp->completed;
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}
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EXPORT_SYMBOL_GPL(init_srcu_struct);
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EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
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EXPORT_SYMBOL_GPL(srcu_read_lock);
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EXPORT_SYMBOL_GPL(srcu_read_unlock);
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EXPORT_SYMBOL_GPL(synchronize_srcu);
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EXPORT_SYMBOL_GPL(srcu_batches_completed);
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EXPORT_SYMBOL_GPL(srcu_readers_active);
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